Horn shark: Difference between revisions

| status_ref = <ref name="iucn status 19 November 2021">{{cite journaliucn | authors author=Carlisle, A.B. |date=2015 |title = ''Heterodontus francisci'' | journal = [[IUCN Red List of Threatened Species]] | volume=2015 | page = e.T39333A80671300 | publisher = [[IUCN]] | year =2015 | doi=10.2305/IUCN.UK.2015-4.RLTS.T39333A80671300.en |access-date=19 November 2021}}</ref>

The French biologist [[Charles Frédéric Girard]] published the first scientific description of the horn shark under the name ''Cestracion francisci'' in 1855, in the ''Proceedings of the Academy of Natural Sciences of Philadelphia''.<ref name="compagno"/> This species was later placed in the genus ''Gyropleurodus'', which was eventually [[synonym (taxonomy)|synonymized]] with the genus ''Heterodontus''. The [[specific name (zoology)|specific epithet]] ''francisci'' is a reference to [[San Francisco]], although the range of the horn shark does not extend that far north.<ref name="ebert">{{cite book |author=Ebert, D.A. |title=Sharks, Rays, and Chimaeras of California |publisher=University of California Press |year=2003 |isbn=978-0-520-23484-70 |pages=81&ndash;86}}</ref> The [[type specimen]] from [[Monterey Bay]] has since been lost. The scientific name for this species has been given erroneously as ''Heterodontus californicus''.<ref name="compagno">{{cite book |author=Compagno, L.J.V. |year=2002 |title=Sharks of the World: An Annotated and Illustrated Catalogue of Shark Species Known to Date (Volume 2) |place=Rome |publisher=Food and Agriculture Organization |pages=36&ndash;37 |isbn=978-92-5-104543-75}}</ref>

Like other bullhead sharks, the horn shark has a short, wide head with a blunt snout and prominent [[supraorbital ridge]]s over the eyes. The horn shark's supraorbital ridges are low and terminate abruptly; the space between them on top of the head is deeply concave. Each eye lacks a [[nictatingnictitating membrane]] and is followed by a tiny [[Spiracle (vertebrates)|spiracle]]. The [[nostril]]s are split into inflow and outflow openings by a long flap that reaches the mouth. The inflow openings are encircled by a groove, while another groove connects the outflow openings to the mouth. The mouth is small and curved, with prominent furrows at the corners. There are 19&ndash;26 tooth rows in the upper jaw and 18&ndash;29 tooth rows in the lower jaw. The teeth at the front of the jaws are small and pointed, with a central cusp flanked by a pair of lateral cusplets; those at the sides of the jaws are much larger, elongated lengthwise, and molar-like.<ref name="compagno"/><ref name="ebert"/>

The horn shark inhabits the [[continental shelf]] of the eastern [[Pacific Ocean]], occurring off the coasts of [[California]] and [[Baja California]] from [[Monterey Bay]] southward, and in the [[Gulf of California]]. Uncommon influxes of warm water northward may bring it as far as [[San Francisco Bay]].<ref name="compagno"/> There are unconfirmed reports of this species off [[Ecuador]] and [[Peru]], which may be misidentifications of other species.<ref name="iucn status 19 November 2021" />

Unlike most fishes, theThe daily activity pattern of the horn shark is under [[exogenous]] control, meaning that it is regulated by environmental factors rather than by an internal physiological cycle. Observations of captive horn sharks show that the relevant cue is light intensity: the sharks become active immediately after the lights are turned off, and stop as soon as they are turned back on. In one experiment where the sharks were kept in darkness, they remained continuously active for 11 days before slowing, possibly from fatigue. In nature, horn sharks exposed to a bright light at night may stop swimming and sink to the bottom.<ref name="nelson and johnson">{{cite journal |title=Diel Activity Rhythms in the Nocturnal, Bottom-Dwelling Sharks, ''Heterodontus francisci'' and ''Cephaloscyllium ventriosum'' |author1=Nelson, D.R. |author2=Johnson, R.H. |journal=Copeia |volume=1970 |issue=4 |date=December 12, 1970 |pages=732&ndash;739 |doi=10.2307/1442315 |jstor=1442315 |publisher=American Society of Ichthyologists and Herpetologists}}</ref>

The horn shark is preyed upon by larger fishes and the [[northern elephant seal]] (''Mirounga angustirostris''), which consumes adults, juveniles, and egg cases. In addition, they are captured and eaten by [[bald eagle]]s (''Haliaeetus leucocephalus'') at Catalina Island, and large marine snails are able to drill into their egg cases to extract the [[yolk]].<ref name="martin"/> The tough skin and spines of this species confer some protection; a [[Pacific angelshark]] (''Squatina californica'') has been filmed engulfing a juvenile horn shark, only to spit it out due to its spines.<ref name="compagno"/> Known [[parasite]]s of this species include the [[tapeworm]]s ''Acanthobothrium bajaensis'' and ''Acanthobothrium puertecitense'', the [[copepod]] ''Trebius heterodonti'', and the [[nematode]] ''Echinocephalus pseudouncinatus'', which spends its [[larva]]l stage inside potential prey such as [[scallop]]s and [[sea urchin]]s.<ref name="Appy1973">{{cite journal |title=Two New Species of ''Acanthobothrium'' (Cestoda: Tetraphyllidea) from Elasmobranchs of the Eastern Pacific |author1=Appy, R.G. |author2=Dailey, M.D. |journal=The Journal of Parasitology |volume=59 |issue=5 |date=October 1973 |pages=817&ndash;820 |doi=10.2307/3278414 |jstor=3278414 |publisher=The American Society of Parasitologists}}</ref><ref name="Caira2001">{{cite journal |title=Two new species of ''Acanthobothrium'' Beneden, 1849 (Tetraphyllidea: Onchobothriidae) from horn sharks in the Gulf of California, Mexico |author1=Caira, J.N. |author2=Zahner, S.D. |journal=Systematic Parasitology |volume=50 |issue=3 |date=November 2001 |pages=219&ndash;229 |doi=10.1023/A:1012241913722|pmid=11590308 |s2cid=24843398 }}</ref><ref name="Deets1989">{{cite journal |title=Three species of ''Trebius'' Krøyer, 1838 (Copepoda: Siphonostomatoida) parasitic on Pacific elasmobranchs |author1=Deets, G.B. |author2=Dojiri, M. |journal=Systematic Parasitology |volume=13 |issue=2 |date=March 1989 |pages=81&ndash;101 |doi=10.1007/BF00015217|s2cid=45745111 }}</ref><ref name="Anderson2000">{{cite book |title=Nematode Parasites of Vertebrates: Their Development and Transmission |url=https://archive.org/details/nematodeparasite00ande |url-access=limited |author=Anderson, R.C. |publisher=CABI |year=2000 |isbn=978-0-85199-421-05 |page=[https://archive.org/details/nematodeparasite00ande/page/n409 389]}}</ref>

Some 95% of the adult horn shark's diet consists of hard-shelled mollusks (e.g. [[bivalve]]s and [[gastropod]]s), echinoderms (e.g. sea urchins) and crustaceans (e.g. [[crab]]s, [[shrimp]], and [[isopod]]s). To crack their shells, the horn shark generates the highest known bite force relative to its size of any shark, well in excess of other measured species such as the [[spiny dogfish]] (''Squalus acanthias'') and the [[blacktip shark]] (''Carcharhinus limbatus'').<ref name="huber et al"/> One study found the average bite force for this species in the wild to be 95 [[Newton (unit)|N]] with a maximum of 135 N, while under experimental conditions sharks could be induced to bite with over 200 N of force.<ref name="huber et al">{{cite journal |title=Analysis of the bite force and mechanical design of the feeding mechanism of the durophagous horn shark ''Heterodontus francisci'' |author=Huber, D.R., Eason, T.G., Hueter, R.E. and Motta, P.J. |journal=Journal of Experimental Biology |volume=208 |pages=3553&ndash;3571 |year=2005 |doi=10.1242/jeb.01816 |pmid=16155227 |issue=Pt 18|doi-access=free }}</ref> Large horn sharks that feed mainly on sea urchins (particularly the [[Strongylocentrotus purpuratus|short-spined purple urchin]], ''Strongylocentrotus purpuratus'') have their teeth and fin spines stained purple.<ref name="ebert"/>

Other prey items of adults include [[peanut worm]]s, [[sea star]]s, [[cephalopod]]s, and small bony fishes. Juveniles feed primarily on polychaete worms, sea anemones, and small [[clam]]s, and have been known to "pounce" on anemones to bite off tentacles before they can be retracted. Off southern California, horn sharks are known to take advantage of seasonal opportunities. In the summer, diurnally active fishes, in particular the [[Chromis punctipinnis|blacksmith]] (''Chromis punctipinnis''), are especially abundant and are easily captured at night when they lie dormant. In the winter, the sharks scavenge on [[Opalescent Inshore Squid|market squid]] (''Loligo opalescens''), which die by the tens of thousands after their mass spawning event.<ref name="ebert"/><ref name="martin">Martin, R.A. [http://elasmo-research.org/education/ecology/kelp-horn_shark.htm Kelp Forests: Horn Shark]. ReefQuest Centre for Shark Research. Retrieved on June 19, 2009.</ref> Horn sharks hunt mainly using their [[olfaction|sense of smell]].<ref name="buch"/> Although [[electroreception]] certainly plays a role in locating prey, this species has only 148 [[ampullae of Lorenzini]]. This is much fewer than in most other sharks, which may have over 2,000.<ref name="Bullock2005">{{cite book |title=Electroreception |author=Bullock, T.H., Hopkins, C.D., Popper, A.N. and Fay, R.R. |publisher=Birkhäuser |year=2005 |isbn=978-0-387-23192-71 |page=49}}</ref> Like other sharks, the horn shark's teeth are regularly replaced; it takes 4 weeks for a dropped tooth to be replaced.<ref name="Reif1976">{{cite journal |author=Reif, W. |year=1976 |title=Morphogenesis, pattern formation and function of the dentition of ''Heterodontus'' (Selachii) |journal=Zoomorphologie |volume=83 |pages=1–47 |doi=10.1007/BF00995429|s2cid=44239227 }}</ref>

The horn shark captures prey via suction, created by expanding its [[buccal cavity]]. Its [[lip|labial]] [[cartilage]]s are modified so that the mouth can form a tube, facilitating the suction force. Once the prey is drawn into the mouth, it is secured with the sharp front teeth and then ground into pieces by the flat lateral teeth. To extract buried or affixed prey, the horn shark grips it and adopts a vertical posture with the head and pectoral fins against the substrate and the tail arched above. The shark then acts as a lever with its pectoral fins as the [[Fulcrum (mechanics)|fulcrum]]: with a downward stroke of the tail, it forces its head upwards and pulls the prey loose; this mode of feeding has not been observed in any other shark. The horn shark is also capable of protruding its upper jaw up to 15% the length of its head; this motion takes only 20 milliseconds to accomplish and allows the shark to use its upper jaw like a [[chisel]] to dislodge firmly attached prey.<ref name="edmonds et al">{{cite journal |title=Food capture kinematics of the suction feeding horn shark, ''Heterodontus francisci'' |author=Edmonds, M.A., Motta, P.J. and Hueter, R.E. |journal=Environmental Biology of Fishes |volume=62 |pages=415–427 |year=2001 |doi=10.1023/A:1012205518704 |issue=4|s2cid=21860117 }}</ref>

[[File:Horn shark egg morro bay.jpg|thumb|left|The spiral-flanged egg case of a horn shark<ref>{{Cite book |last=Freeborn |first=Michelle |url=https://www.wikidata.org/wiki/Q58012425 |title=The fishes of New Zealand |date=2015-01-01 |publisher=Te Papa Press |isbn=978-0-9941041-6-8 |editor-last=Roberts |editor-first=Clive Douglas |volume=2 |pages=54 |editor-last2=Stewart |editor-first2=Andrew L. |editor-last3=Struthers |editor-first3=Carl D.}}</ref>; the shape allows the egg to be secured within crevices.]]

Mating in the horn shark occurs in December or January, on a possibly annual reproductive cycle.<ref name="fowler et al"/> The male chases the female to indicate interest; once she is ready both sharks settle on the bottom, where the male grips the female's pectoral fin in his teeth and inserts one of his [[clasper]]s into her [[cloaca]]. After 30&ndash;40 minutes of [[copulation (zoology)|copulation]], the pair disengages and the female spins with her snout in the sand for another 30 minutes.<ref name="martin"/> From February to April, the females lay a maximum of 24 eggs two at a time once every 11&ndash;14 days, in water {{convert|2|-|13|m|ft|abbr=on}} deep.<ref name="iucn status 19 November 2021" /> The egg case has two flanges spiraling around it, and thus may take the female several hours to deposit.<ref name="Ref_">Martin, R.A. [http://elasmo-research.org/education/shark_profiles/heterodontiformes.htm Heterodontiformes: Bullhead Sharks]. ReefQuest Centre for Shark Research. Retrieved on June 19, 2009.</ref> At first the case is soft and light brown, and over a few days it hardens and darkens in color. Not including the flanges, the case measures {{convert|10|-|12|cm|in|abbr=on}} long and {{convert|3|-|4|cm|in|abbr=on}} wide; sharks from the Channel Islands produce longer egg cases than those from mainland California, suggesting that they are separate populations.<ref name="compagno"/><ref name="ebert"/>

One of the few sharks to exhibit [[parental care]], female horn sharks in the wild pick up their eggs in their mouths and wedge them into crevices.<ref name="ebert"/> However, in captivity the eggs are simply dropped on the bottom and may later be cannibalized.<ref name="compagno"/> The eggs hatch in 6&ndash;10 months; at emergence the young measure {{convert|15|-|17|cm|in|abbr=on}} long.<ref name="iucn status 19 November 2021" /> Newly hatched sharks are provisioned with an internal [[yolk sac]] and do not have to feed until they are a month old, though they are capable of feeding and will accept food during this period. Horn sharks grow slowly and at a highly variable rate that does not correspond to their size; this has frustrated attempts to determine their aging process.<ref name="ebert"/> Males mature at a length of {{convert|56|-|61|cm|in|abbr=on}} and females at a length of at least {{convert|58|cm|in|abbr=on}}.<ref name="iucn status 19 November 2021" /> Individual sharks have lived to over 12 years old in captivity, and there exists an unconfirmed report of a shark reaching 25 years of age.<ref name="ebert"/>

Under normal circumstances, horn sharks are harmless to humans and can readily be approached underwater.<ref name="ebert"/> However, they can be provoked into biting, and some pugnacious individuals have been known to chase and bite divers after being harassed.<ref name="martin"/> These sharks should be handled with care as their fin spines can inflict a painful wound.<ref name="ebert"/> The horn shark adapts well to captivity and has been maintained and bred in many [[public aquarium]]s across the United States.<ref name="compagno"/> In July 2018, three people were arrested after stealing a juvenile horn shark from the [[San Antonio]] Aquarium. The shark was smuggled out of the aquarium in a stroller under a blanket. It was returned unharmed two days later.<ref>{{Cite web|url=https://abcnews.go.com/US/shark-returned-san-antonio-aquarium-video-shows-brazen/story?id=56929004|title = Shark returned to San Antonio aquarium after being stolen in baby stroller| website=[[ABC News (United States)|ABC News]] }}</ref>

The horn shark has no commercial value in California, where it is captured unintentionally in traps and trawls and by [[recreational fishing|recreational anglers]]. The shark's hardiness ensures that it can often be returned to the water alive.<ref name="iucn status 19 November 2021" /> This species benefits from general restrictions placed on coastal fishing gear by the State of California. The average annual [[bycatch]] off California is {{convert|1800|kg|lb|abbr=on}}, though historically it has varied from {{convert|2.5|kg|lb|abbr=on}} in 1976 to {{convert|9500|kg|lb|abbr=on}} in 1979.<ref name="fowler et al">{{cite book |author=Fowler, S.L., Cavanagh, R.D., Camhi, M., Burgess, G.H., Cailliet, G.M., Fordham, S.V., Simpfendorfer, C.A. and Musick, J.A. |title=Sharks, Rays and Chimaeras: The Status of the Chondrichthyan Fishes |publisher=International Union for Conservation of Nature and Natural Resources |year=2005 |isbn=978-2-8317-0700-56 |pages=237&ndash;238}}</ref> Divers sometimes kill them for sport or to make jewelry out of their fin spines, which may be the cause of a decline in the numbers of horn sharks in the most intensely dived areas of southern California. Off Mexico, this species is caught incidentally in shrimp trawls and demersal [[gillnet]]s, and used for human consumption and fishmeal. The expansion of Mexican gillnet fisheries may pose a conservation concern in the future. At present, the [[International Union for Conservation of Nature]] (IUCN) does not have sufficient information to assess the overall conservation status of this species; its status in United States waters is likely [[Least Concern]].<ref name="iucn status 19 November 2021" />

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{{commonsCommons category|Heterodontus francisci}}

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[[Category:Western North American coastal fauna|Shark, Horn]]

[[Category:Fish of the Gulf of California|Shark, Horn]]